Dewatering at Crane Valley Dam

Seismic retrofitting at Crane Valley Dam on Bass Lake in California required a dewatering system at the downstream toe of this large dam to facilitate a dry excavation. Although the excavation was only 1.5 to 4.6 m deep (but 152 m long), it was of paramount importance to control the artesian pressures, prevent breakout of seepage through the hydraulic fill embankment, and ensure a stable excavation subgrade, while the lake level remains 12 to 20 m higher than the excavation. The designers developed a system comprising ejector wells spaced 3 m on centers with 5 deep wells for the dewatering system. The system performed extremely well. Copious data was obtained on the flow rates in the ejectors and individual deep wells, piezometric levels in numerous piezometers, lake level and precipitation. The system remained in operation for over 10 months. This paper describes the interpretation of the data, groundwater modeling, the parameters controlling the design of the system, and the influence of the actual lake level and rainfall on the operation of the system.